Raman Amplifiers

Abstract

A pump unit ( 402 ) for a Raman amplifier ( 400 ) including an optical fibre ( 401 ) carrying an optical signal ( 420 ) is disclosed. The pump unit includes at least two light sources ( 411, 412, 431, 432 ) for emitting light at different wavelengths into the fibre to induce Raman gain of the optical signal passing along the fibre, and a controller ( 409 ) for providing pulses to each of the light sources to control when they do and do not emit light. The controller is configured to control the width of the pulses to control the total power of the light emitted into the fibre.

1 . A pump unit for a Raman amplifier having an optical fibre carrying an optical signal, the pump unit comprising:
 at least two light sources for emitting light at different wavelengths into the fibre to induce Raman gain of the optical signal passing along the fibre; and   a controller for providing pulses to each of the light sources to control when they do and do not emit light,   wherein the controller is configured to:
 control the width of the pulses to control the total power of the light emitted into the fibre, and 
 optimise overlap times during which the light sources are activated simultaneously so that the overlap time between the light sources is minimised when light from one light source falls near the peak of a Raman gain spectrum produced from light of another light source. 
   
     
     
         2 . The pump unit according to  claim 1 , wherein the controller comprises a pulse width modulation, PWM, unit for varying the width of the pulses. 
     
     
         3 . The pump unit according to  claim 1 , wherein the controller is configured to vary the duty cycles of the pulses to each of the light sources in response to changes in gain conditions, bandwidth and/or channel allocation in the amplifier. 
     
     
         4 . The pump unit according to  claim 1 , configured to allow a long overlap time between two light sources when light from the two sources does not interact strongly. 
     
     
         5 . The pump unit according to  claim 1 , wherein each of the light sources is configured to emit light at a high pump power. 
     
     
         6 . The pump unit according to  claim 1 , wherein each of the light sources is configured to operate in multi longitudinal mode. 
     
     
         7 . The pump unit according to  claim 1 , wherein each of the light sources is configured to operate in coherence collapse mode. 
     
     
         8 . The pump unit according to  claim 1 , wherein the controller and the light sources are provided in an integrated package. 
     
     
         9 . The pump unit according to  claim 1 , wherein each of the light sources is configured to emit counter-propagating light travelling in the opposite direction to the optical signal passing along the fibre. 
     
     
         10 . The pump unit according to  claim 1 , wherein each of the light sources is a laser. 
     
     
         11 . A Raman amplifier assembly having an optical fibre carrying an optical signal, the assembly comprising:
 at least two light sources for emitting light at different wavelengths into the fibre to induce Raman gain of the optical signal passing along the fibre; and   a controller for providing pulses to each of the light sources to control when they do and do not emit light, wherein the controller is configured to:
 control the width of the pulses to control the total power of the light emitted into the fibre, and 
 optimise overlap times during which the light sources are activated simultaneously so that the overlap time between the light sources is minimised when light from one light source falls near the peak of a Raman gain spectrum produced from light of another light source. 
   
     
     
         12 . The Raman amplifier assembly according to  claim 11 , further comprising a pump unit according to  claim 1 . 
     
     
         13 . A method of controlling a pump unit used in a Raman amplifier system having an optical fibre for carrying an optical signal, the method comprising:
 emitting light at different wavelengths into the fibre to induce Raman gain of the optical signal passing along the fibre by means of light sources;   providing pulses to each of the light sources to control when they do and do not emit light; and   varying the width of the pulses to control the total power of the light emitted into the fibre, and   optimising overlap times during which the light sources are activated simultaneously so that the overlap time between the light sources is minimised when light from one light source falls near the peak of a Raman gain spectrum produced from light of another light source.   
     
     
         14 . A computer program, comprising computer readable code which, when run by a unit, causes the unit to perform the method of  claim 13 . 
     
     
         15 . A computer program, comprising computer readable code which, when run by a controller of a pump unit, causes the pump unit to operate as the pump unit of  claim 1 . 
     
     
         16 . A computer program product comprising a computer readable medium and a computer program according to  claim 14 , wherein the computer program is stored on the computer readable medium. 
     
     
         17 . A computer program product comprising a computer readable medium and a computer program according to  claim 15 , wherein the computer program is stored on the computer readable medium.